Thursday, November 4, 2010

Bug Rescue Part 2: Dragonflies are Way Cool

After my ride I showered, dressed and drove into the office. At about noontime I walked out the back door of our building, by the overhang and the bench where the smokers hang out, to run some errands. On the walkway about 10 feet from the door was this dragonfly, just sitting there. It was cool out, but not cold, so I wasn’t clear why it was sitting there. I picked it up while it flapped feebly (Dragonflies really can’t walk, as we’ll see in a moment) and placed it on the grass, thinking at least to get it out of the way of foot traffic. I sprinted back up the back stairs for my camera, returned to find her still in place, and snapped these photos.

Tangent: Right by the back door of my office building is, as I mentioned, a little overhang with a bench where smokers hang out. Two of my coworkers- let’s call them “Aaron” and “Jimmy”*- are smokers, and I’ll frequently stop to say hi to one or the other on my way in or out of the building. Sometimes they’ll be chatting with other smokers, and other times just taking a break alone, quietly looking out across the creek, or toward the mountains in the background. And, I have to admit that deep down, I am a teeny bit envious. Smoking gives them an excuse to go outside, sit on a bench on a nice day and, for a few minutes, do nothing. Oh, I get that smokers also have to huddle outside on freezing days, but here in Utah it seems that there are, on the balance, more nice than bad days to pass a few minutes out-of-doors.

*Not to be confused with “Fast Jimmy”, who does not smoke, and is not a coworker.

Of course we non-smokers can also take mini-breaks, but since we don’t have an “excuse”, we’re more furtive about them, and spend them staring at our screens, pretending to work, while we read blogs or Dear Prudence* or whatever.

*I always thought I’d make a great advice columnist. How do I get that job?

Office smokers also tap into a little social network that we non-smokers are largely cut off from. There are probably a dozen other companies represented in our building; I don’t know a soul from any of them*. But Jimmy and Aaron are often chatting easily with smokers from several other companies, and so, oddly, smoking has expanded their world.

*Except that crazy older woman with the spiky red hair who always tries to start up a conversation with me in the elevator. Or did, anyway. Now I always take the stairs.

Nested Tangent: It’s probably expanded the world of Aaron- who is single- even more, as several of the smokers over the years have been attractive young women. For a while I teased Aaron about this, as I often came across him smoking/chatting with 2 particularly striking young women who worked at the multi-level-marketing company downstairs*, and whom I referred to- in what I always felt was a particularly inspired bit of double-entendre- as “the Smoking Hotties.”

*Special Footnote for Non-Utah Readers: Every office building in Utah contains a multi-level marketing company. The products touted are almost always health/wellness-related, which is kind of ironic in that most of the people who actually work at these companies, uh, don’t look all that healthy.

Yes, I know that Aaron and Jimmy will likely pay for their pleasant downtime-breaks and expanded social world by, well, you know, dying horrible premature deaths and what-not, but that’s not the point. The point is- and I do have one- why do you need an excuse to go hang outside for 5 minutes in the middle of the day? If I just went out and sat on the bench and a coworker passed and saw me doing nothing, they’d probably think it a bit odd. But if I were smoking, well then that would be totally fine…

The order Odonata, which includes both dragonflies and damselflies, includes some 5,500 species across all continents except Antarctica. The big obvious difference between dragon and damselflies BTW is the resting position of the wings. If it folds them back when at rest, it’s a damselfly; if it keeps them out at 90 degrees to the body, it’s a dragonfly.

Extra Detail #1: They also exhibit different mating flight patterns. Dragonflies generally mate while flying. Damselflies spend more of their mating time perched, but often fly- while connected- for short distances from perch to perch.

Tangent: Every once in a while, you’ll see or hear about an animal doing something that looks really fun- jumping out of the water, swinging between tree limbs, flying rapidly through the air. I just want to point out that mating while flying sounds like about the funnest thing imaginable.

Extra Detail #2:Odonata appears to be monophyletic, meaning the group includes all of the descendants form a common ancestor. Dragonflies (infraorder Anisoptera) also appear to monophyletic. But damselflies (Infraorder Zygoptera) appear to be paraphyletic, in that one big genus (Lestes) turns out to be more closely-related to dragonflies than to any other damselflies.

This sounds kind of geeky, but it means that “Damselfly-ness”, specifically the hinged-wing mechanism, has to have evolved at least twice among the odonates, a cool example of convergent evolution.

The dragonfly I rescued is one of the most common species in North America, and if you’ve ever paid attention to Dragonflies you’ve almost certainly noticed it. It’s the Common Green Darner, Anax Junius, and its distinctive green color is a quick identifier, as is the apparent “bulls-eye” atop it’s forehead/“nose” when viewed from above.

This one was most likely female, because the main part of the eyes are brown-hued. The long abdomen, consisting of 10 segments, is brown or dull/dark purplish in females, but often a bright blue in males. While we’re on the topic of gender, this is probably a good time to talk about the lifecycle of Dragonflies.

If you know anything about dragonflies, you probably have heard that they develop through an aquatic “nymph” stage (pic right, not mine) before emerging from the water and developing fully into flying adults. I’ve known this for a long time, and always thought of dragonflies, and other insects with an aquatic nymph stage, as sort of “amphibious bugs”; they spend a little time in a “child” stage in a pond, before getting on with, you know, their “real” life. But when I started learning about them, what really surprised me about dragonflies was how much of their lives- the vast majority in fact- is spent in the nymph stage. Green Darners commonly live as nymphs for up to 2 years, and other species live as nymphs for as long as 5 or 6 years.

But an adult darner lives only for a couple of months. Think about how weird this is. We think of animals developing like mammals and birds and reptiles generally do- we’re born, we grow up quickly, and spend the majority of our lives in sexually mature adult form. But what if you were born, grew to about the size of an 8 or 9 year old, and then stayed that way for like 60 years? Then, right around when you collected your first social security check, you suddenly hit puberty, grew armpit hair and got interested in the opposite sex. But you had to hustle, because you only had a few years or so to marry and have kids before you dropped dead! That’s pretty much what the lifecycle of dragonflies (and many other insects) is like…

A dragonfly’s life, therefore, is mainly a nymph’s life, which most of us never see. Dragonfly nymphs are fearsome aquatic predators. How fearsome? Did you ever see any of the Alien Movies?*

*I love the original Alien. The various sequels never worked for me. But I loved the original. I’ve seen it like 10 times, and every time where it gets to the part where Sigourney Weaver goes back for that cat, I’m always yelling at the TV, “Screw the cat! Just get out of there!”

The super-scary alien-predator in the Alien movies had several fearsome weapons- spiked tail, long claws, acid blood, etc. But the most terrifying was its extendable jaw. Remember that? It would get real close to you, open its wide jaws menacingly, all fangs and drool, and then all of sudden, another set of jaws would jump out from inside its mouth and like rip your head off! Wasn’t that scary? Well that, more or less, is exactly the deal with a dragonfly nymph!

The extending jaw of a dragonfly nymph isn’t positioned like the Alien jaws (i.e. it’s not inside the mouth) but rather the lower jaw, the labium (diagram left, not mine, and below, not mine either) is hinged and extendable. As the nymph closes on its prey, the labium jets out lightning fast, clamps down on the prey, and yanks it backward into its maw. I’d think that if you’re a mosquito larva or a small tadpole, it’s about the scariest thing imaginable.

Nymphs molt between 6 and 15 times before climbing up the stem of an emergent plant* above the surface of the water, where they molt a final time, emerging at last as a winged adult. This final molt, BTW, which is followed by a wing-drying period, is one of the most vulnerable times of a dragonfly’s life. Although they’re fearsome hunters, they’re also regularly preyed upon by everything from birds to frogs.

*I explained emergent plants in this post. Man, it is like I have a post for everything.

As aerial predators, dragonflies feed upon all sorts of mosquitoes, midges, gnats, flies and other insects. Their 6 spiked legs, held in a basket-like formation as they fly, are used to scoop up prey toward the mouth.

In fact, scooping and perching are more or less the only things a dragonfly can do with its legs; they can’t really walk, and so if unable to fly are pretty much immobilized.

Male Green Darners, like most male dragonflies, are territorial, and patrol their territory getting into tussles with interlopers, and looking for females. The male produces a sperm packet from the tip/10th segment of his abdomen, and then curls his abdomen under itself to deposit the packet in a small depression on the underside of his 2nd abdominal segment.

He then flies out and grabs the head of a female in mid-air, with the clasping genital tip of his 10th segment. Then, connected tip-to-head, the two fly around together for a while, in the dragonfly version of foreplay. Usually the male does the flying and just pulls the female along for the ride, but every once in a while the female might flap a for a bit while the male rests.

After some period of time, the female bends her abdomen below her and up under the male’s abdomen to pick up the sperm packet with her genital opening. They fly around in this “wheel” position together for up to 15 minutes.

After copulation, different species do different things. A female Twelve-Spotted Skimmer, for example, promptly disengages from the male and speeds off on her own to lay eggs on the surface of a nearby body of water. But Green Darners remain connected and fly to the water’s surface together, repeatedly, to lay eggs in multiple locations.

Extra Detail: Dragonflies use the same XX/X0 chromosomal system of sex determination used by Fruit Flies, which I described in last year’s Housefly series*. I had a hell of a time BTW determining the chromosome # of A. junius. I believe the diploid # is 27 (male)/28 (female) but this could be wrong.

*Man, was that an awesome series or what?

Damselflies, BTW, remain connected for egg-laying, but take it a step further. The pair lands on an emergent plant stem, then crawls down together- still connected- beneath the water’s surface where the female deposits the eggs into the stem of the plant. It’s suspected that the pair may do this together because the female requires the added strength/mass of the male to break the surface tension of the water and re-emerge into the air.

One of the reasons dragonflies fascinate me is that, like sharks or scorpions, it’s a really old design that’s held up amazingly well. Think about it. Tool-using hominids have been floundering along for maybe a couple of million years and already are in the midst of a wacky population explosion/environment-world-alteration that’s anybody’s bet as to whether they’ll survive it. Dragonflies have been around for at least 325 million years, and were not only one of the first types of insects to evolve flight, but were part of the teeny fraction of living creatures to survive the Permian extinction. And here they are today, thriving all over the world, with largely the same structure and hardware.

One great example is the eye, which I won’t cover in detail here, because we covered it in the run-down of compound eye types I posted about in the Housefly series. You can check out that post for details, but the key take-away is that the eye of the Dragonfly, an apposition compound eye, is the simplest and most ancient compound eye “design” there is. Compared to the eye of a housefly, moth or lobster, it’s downright primitive. Yet dragonflies are stunningly successful visual hunters.

Another example, one that also contrasts with the housefly, is wings. The wings of a housefly are highly-advanced, with the 2 rear ones having evolved into sophisticated tiny flight stabilizers called halteres. A dragonfly by contrast is equipped with the same primitive 4-independent-wing system common to the earliest flying insects.

Side Note: I should mention that the whole topic of the evolution of wings and flight in insects is one of the big stumpers in evolution. There are lots of ideas, but still no consensus. With something like a bird or a bat or a pterosaur, even if we can’t figure out exactly how it evolved flight, it’s pretty obvious where the wings came from- the forelimbs. But there’s not an equally obvious wing-precursor-limb in insects. One thing that does seem apparent is that insects don’t seem to have been particularly successful or abundant before evolving flight…

Yet dragonflies are awesome fliers. They’re fast and maneuverable, rapidly changing direction and accelerating on a dime to speeds of >60MPH. They can also fly backwards (though at only around 3% their maximum forward speed), and hover in place for up to a minute.

Extra Detail: Know why they can’t hover longer? Because they overheat, which makes perfect sense when you think about it. Dragonflies do most of their flying at significant speeds, experiencing fast, cooling airflow. In the absence of that airflow, they run hot. Plus, their tracheal respiratory system- to which we will return momentarily- is more efficient in strong airflow.

Dragonfly wings, though ancient in form, turn out to be remarkably sophisticated. They consist of largely clear membranes held in place by a network of veins. All dragonfly and damselfly wings have 5 primary veins. Some veins are darker and thicker than others, and these support portions of the wing that experience greater stress during flight. The wing has a notch/vein-junction on the forward edge called the nodus, that is critical to the strength and structural stability of the wing.

The wing does not form a smooth surface; if you run your fingertips across the surface of the wing*, it feels corrugated. But these corrugations aren’t random; their pattern optimizes airflow in a way that combines the advantages of a flat surface with an airfoil. Dragonfly wings cut through the air with minimal drag, making odonates some of the relatively few insects that are excellent gliders. Yet the wing, despite having no real curvature**, exhibits awesome lift properties. Human engineers haven’t developed anything like the dragonfly wing.

*Only do this with an already-dead dragonfly. Man-handling the wings of a live dragonfly will likely mean its early demise.

**I should say “consistent curvature”. It could be argued that the wings exhibit a type of cumulative effective curvature. See sources for more details.

Another cool feature of dragonfly wings is the teeny-weeny little opaque/colored patch on the front edge of the wing by the tip. I’d noticed these patches in the past and assumed they were simply decorative. But the patches, called pterostigmae, are highly functional. The pterostigma is a region of much denser cells that increases the mass of that portion of the wing and adds significant gliding stability. Gliding with a long, light wing is apparently more stressful than you might think, and the presence of these weighted patches is estimated to improve gliding efficiency by some 10-25%.

Side Note: While we’re on the topic of wings, it’s worth noting that these structures and characteristics of Dragonfly wings have been around for a very, very long time, even back in the Carboniferous period (300 – 360 MYA) (artsy conception right, not mine), when dragonflies grew far larger than they do today, with wingspans of up to 28 inches. That implies that these huge dragonflies were still incredibly agile fliers, which would’ve been awesome to see, though probably a bit un-nerving were one to buzz close by…

The presumed reason for large size was the greater concentration of oxygen in the atmosphere in the Carboniferous, likely over 30%*. The increased O2 levels would have made the tracheal respiratory system of insects** more efficient, allowing them to grow- and fly- at much larger mass.

*Which is higher than the concentration necessary for wet wood to burn, and makes you wonder about the forest fires in those times…

What’s interesting about oxygen and bugs in the Carboniferous though is that although many other bugs grew much larger than they did today, not all of them did. Cockroaches for example, which also were around at that time, did not grow particularly large. In fact, I believe(?) that the largest cockroaches that have ever lived are around today. In recent experiments researchers have raised dragonflies*, roaches and other bugs under hyperoxic (high oxygen level) conditions, and found that while dragonflies, and most other insects, grew bigger and faster than they do at normal O2 levels, the roaches grew at roughly half the rate the do otherwise, and their tracheal tubes were abnormally small.

*Dragonflies are apparently a real bitch to raise in captivity. Feeding them is the problematic part…

So what was my lady dragonfly doing just laying around on the sidewalk? Maybe she was old and about to check out. Maybe she was out hunting and nightfall caught her out and about. Or maybe, just maybe, she’d spent the night in unfamiliar country, in the middle of a migration.

Out of thousands of dragonfly species, only a few dozen are known to be migratory. The Green Darner is one of those, but much about its migration behavior is still unknown. And in some respects, the more that becomes known, the more confusing the species and its behavior becomes.

In the Eastern and central US, Green Darners are known to form huge swarms, numbering up to over a million, migrating South in the Fall. Their paths and destinations are not completely known, but swarms have also been reported in Central America, suggesting crossings of the Gulf of Mexico. Such crossings, if they do occur, are certainly possible. Although the maximum fat-reserve slight time of a Green Darner is thought to be only about 8 ½ hours, migrating dragonflies often feed while on long-distance journey, taking sustenance from what is known as “aerial plankton”, including tiny aphids, midges and spiderlings aloft in the sky.

Extra Detail: The ~8 hour estimate comes from analysis of dragonfly body fat, which can account for up to 30% of body mass. Whoda thunk?

Dragonfly migration, like that of monarch butterflies, is multi-generational; the dragonflies who fly South in the Fall are not the same individuals who fly North in the Spring. Long-distance migration is always awesomely impressive, but multi-generational long-distance migration even more so. How on Earth do they do it? Does every dragonfly have a built in instinctive geographic map and awareness of the world? Do they crawl up out of the swamp, do a final molt, and think, “Oh hey, looks like I’m in Belize. Guess I better start flying to Ontario…”? That’s hard to swallow. Insect brains are teeny-weeny-tiny, and in dragonflies, something like 80% of that teeny brain is believed to be devoted to visual processing. It seems unlikely that they could possess anywhere near that level of geographic self-awareness,

To try and better understand dragonfly migration, researchers in the Fall of 2005 captured 14 Green Darners (1/2 male, ½ female), equipped them radio transmitters, and then monitored their positions for an average of 6 days each. What they found was that migratory flight appeared to follow simple, predictable rules. For example, the dragonflies migrated Southward roughly every 3 days. A Southward-flying day always occurred when the previous night was colder than the night before. Migration days tended to occur on days with lower windspeeds, and no dragonfly was observed migrating on any day where the winds gusted to over 16 mph. Winds were most often Northerly on migration days. Many of these behaviors are remarkably similar to migrating songbirds, who regularly mix up migrating and “stop-over” days over the course of their annual migrations.

In other words, the behavior suggested that a dragonfly brain follows simple, almost Boolean, rules in migration and navigation, which might explain how something so small-brained might accomplish such impressive long-distance navigation and migration.

Tangent: There’s a wonderful analogy here that I can’t resist. Remember the Life Reference Architecture tangent from the Triangle Man post? It’s like the Dragonfly brain contains a “Migration Decision Point”, a set of Boolean, context-driven rules, which guide its decisions in that part of its life. I wouldn’t be surprised if dragonfly behaviors in other areas- hunting, mating, predator-evasion- could be similarly encapsulated in Boolean format. Maybe the Reference Architecture is an effective analogy for how an insect brain works- a set of Decision Points that, in simple form, drive apparently complex context-based decisions.

Green Darners don’t always migrate in swarms (and to my knowledge swarms don’t occur in the Western US). Sometimes they migrate solo, though again, it’s not clear how far or where. But fascinatingly, many Green Darners don’t migrate. Instead they over-winter, and do so in really cold places all over the US and Southern Canada. They over-winter not as adults, but as nymphs, in sort of a diapause, or delayed developmental state, under the ice in frozen ponds, wetlands, etc.

So if Green Darners can over-winter in cold climes, why migrate? A decade ago dragonfly researcher Philip Corbet suggested that maybe the migrants and non-migrants represented 2 distinct subspecies of Green Darner. Such an explanation would make sense. Maybe the non-migrants were Darners who had figured out how to survive the Northern winters, and were on their way to forming a new species of dragonfly. But subsequent research seems to have debunked the subspecies hypothesis. DNA analysis of nearly 100 Green Darners, both migrants and non-migrants, collected across North America revealed several distinct genetic lineages.

Side Note: This kind of lineage-analysis has been done with lots of creatures, including humans. You’ve probably heard of the “Mitochondrial Eve”, the presumed most recent common female ancestor of all people alive today. Subsequent research has suggested all sorts of more recent maternal and paternal lineages all over the world. A fascinating example is described in Brian Sykes’ Seven Daughters of Eve*, which details research around the seven maternal lines from which the vast majority of Europeans appear to be descended within the last ~55,000 years or so.

*The concept and research of the book is fascinating. The fictional what-if chapters were a little less compelling for me.

What researchers found was that both migrants and non-migrants existed in multiple separate Green Darner lineages, meaning that non-migratory (and/or migratory) behaviors had apparently come about repeatedly and independently, suggesting a significant degree of “plasticity” in migratory tendencies across the species.

By now you’re probably getting an idea of why I characterized Green Darner migration as confusing. I don’t know where my lady dragonfly came from, or how she wound up on the walkway by the smoker’s hang-out. But I’m glad I stopped to check her out. Second bug rescued.

6 comments:

MLM companies: So I can blame Utah for this? Just last week, a former classmate asked me to talk to a "friend" of his about the friend's business idea. I said sure. Turns out it was an hour-long pitch for a MLM "wellness" company... although in fairness it was out of Idaho, not Utah.

Dragonflies hitting puberty in their relative 60's seems extreme, but mayflies take it a step further. Mayflies spend most of their life in nymph stage, emerging from the water just long enough to mate and, for the females, lay eggs. Then they die. They emerge with no mouths, as they do not feed during the adult stage--it lasts a matter of hours. Hope the flying sex is really, really good, because they're not getting much of it.

But damselflies (Infraorder Zygoptera) appear to be paraphyletic, in that one big genus (Lestes) turns out to be more closely-related to dragonflies than to any other damselflies.

Bybee et al. (2008) re-examined this using a much larger data set than the Saux et al. study and recovered a monophyletic Zygoptera. The idea of a paraphyletic Zygoptera seems to have been the result of inadequate sampling.

it means that “Damselfly-ness”, specifically the hinged-wing mechanism, has to have evolved at least twice among the odonates

Damselfly wings aren't hinged. Damselflies can't actually fold their wings 'back' any more than a dragonfly can; like a dragonfly, their wings only move up and down (though they do move over a much greater arc than dragonfly wings). Instead, damselflies have developed a majorly angled back, so that the part with the wings has become almost vertical instead of horizontal, so 'up and down' for the wings has nearly become 'forwards and backwards'. Compare this to most other insects that are able to rotate the whole wing rearwards flat over the back.

Offhand, Epiophlebia, the only living odonate that is not either a dragonfly or a damselfly, moves its wings in a damselfly-like arc despite being otherwise more like a dragonfly. So the ability to move the wings damselfly-wise is the ancestral condition for odonates; the specialised condition is the dragonfly arrangment of having them permanently outwards.

Phil- I'm pretty sure Patty or Selma (not sure which one) used to work at our office until about a year or so ago. (Lurking coworker readers know exactly who I'm talking about.)

SBJ- Yes, mayflies do take it to the extreme. Maybe having 2 penises helps them get the most out of their fleeting adulthood.

Christopher- Thanks for the update and pointer to the Bybee paper, as well as as the damselfly wing explanation. I'll check them (damselflies) out more closely next opportunity (which I fear may be next spring now).